Most security apparatus falls in one of two broad classes. The first of those classes is formed by locks and fences and other apparatus whose purpose is to physically prevent a violation of security. In the second class falls that apparatus which includes a sensing means to detect an actual or potential security violation and which generates a signal which is used either in attempting to deter the violator or to facilitate his identification and capture.
This second class of apparatus is usually intended to deal with one of three kinds of security violation. Thus, some apparatus and systems are concerned with intrusion into a place that is to be secured. Some apparatus of this second class is employed to prevent theft of apparatus. Most of the remainder of the security hardware in this class is used to protect individuals against attackers.
To a large extent the sensors employed in this second class of material are specific to a particular task. A wide variety of approaches have been used. In some cases switches are opened and closed. Motion is sensed. Some involve measurement of echoes of sonic or radio signals. The use of different principles and cost considerations have led to a wide variety of very specialized apparatus each with a limited purpose. Change in acceleration sensors, which are usually called "motion sensors" in this art, are usually capable of detecting motion only in two dimensions. In cases where detection of security violation is used to deter the violation, the theory of the apparatus is to sound an audible alarm as loudly as the available power and cost permit.